1. Field of the Invention
The present invention relates to a method of continuously measuring the flow of a liquid. Pumping liquids often results in the generation of bubbles which make it difficult to measure and control with conventional flow measuring devices.
2. The Relevant Technology
In the field of gaseous phase sterilisation the most common method to generate the vapour is by evaporating an aqueous solution of the sterilant on a hot surface. Typically, the solution would be 35% w/w hydrogen peroxide. Such solutions tend to be unstable and give off bubbles of gas, which interfere with the conventional flow measuring systems.
This problem was recognised and dealt with in EP O 662 844 B1, by drawing the sterilising solution from a container into an accumulator and measuring the weight loss in the container. Whilst this technique allows for a known weight of liquid to be delivered to the evaporator and then to turn this weight of liquid into a vapour it has two shortcomings. Firstly, it is necessary to decide at the start of the process how much liquid will be required, and secondly it limits the amount of liquid that may be dispensed. The apparatus described in EPO 662 844 B1 requires that the liquid flow rate to the evaporator is greater than the flow rate used to fill the accumulator. This difference in the flow rate means that the accumulator cannot be refilled from the container as the flow from the accumulator will always be greater than the flow with the accumulator.
It is also important in gaseous sterilisation process to be able to control the concentration of the gas being delivered to the chamber to be sterilised. The concentration will depend on the mass flow of the carrier gas, normally air, and the rate at which liquid is evaporated into the air stream. The present invention not only deals with the first two difficulties found in Patent EPO 662 844 B1 but also provides a method of measuring and controlling the liquid flow to the evaporator.